1. Field of the Invention
The present invention relates to a method of low-NOx combustion and a burner device for effecting the same. More particularly, the invention is directed to an improvement of a two-staged low-NOx combustion method and a two stage firing burner device for carrying out the method.
2. Description of Prior Art
Among various conventional low-NOx combustion methods, there has been known a two-staged method comprising two fuel supply stages for doing the combustion at two stages, as disclosed, for instance, from the Japanese Patent No. 1104160. (Hereinafter, this method will be referred to as "two-stage fuel combustion method".) Such two-stage fuel combustion method is normally executed by a burner device as shown in FIG. 1. According thereto, there is provided a burner device BD" which has a burner throat 103 formed therein and one piece of primary fuel nozzle 101 disposed within the burner throat 103. Further, a plurality of secondary fuel nozzles 102 are provided around the outside opening of the burner throat 103. Each of those secondary fuel nozzles 102 is oriented toward a primary flame which is to be flowed out from the burner throat 103. With this device, a whole amount of combustion air (as designated by "Air" in FIG. 1) is supplied in the throat 103, and a primary fuel is injected from the primary fuel nozzle 101 toward the combustion air, such that the primary fuel is embraced or circumscribed by the air, to thereby effect a combustion and create the primary flame. Then, in the vicinity of the opening of burner throat 103, a secondary fuel is injected from the secondary fuel nozzles 102 toward the thus-created primary flame, creating thus a secondary flame. Namely, in this sort of combustion method, the first combustion stage uses the whole amount of combustion air to burn the primary fuel under a proper excess air condition set by an suitable excess air ratio (i.e. the so-called "air rich" condition), and then, the secondary fuel is injected to such first combustion, reducing a part of NOx existing in the primary flame and thereafter bringing the primary fuel in contact with the downstream portion of combustion air which remains not burned through the primary flame, so as to effect a second combustion, creating a secondary flame.
However, the above-described conventional method and burner device inject out the combustion air from the burner throat 103, in such a way that the primary flame is surrounded by the air, which has been found defective in that the combustion air, which flows in the thin-arrow direction in FIG. 1, results in expanding its stream at the exit of burner throat 103 as indicated by the arrow A.sub.2, and the expanded portion of air directly contacts the secondary fuel injected from the secondary fuel nozzles 102, causing a combustion in this particular area. Hence, a part of the secondary fuel is directly contacted with such leaked air (A.sub.2) before contact with the primary flame, starting thus a secondary combustion in advance. Consequently, the combustion air is not fully used to reduce the NOx in the primary flame and there is a problem of insufficient NOx reduction. Although this prior-art technique serves the low NOx purpose based on the thick and thin fuel combustion principle, more effectively than most of ordinary combustion techniques, yet there is a room of improvement for the reason above.